It is known through clinical experience extending over several decades that titanium and its dilute alloys have requisite biocompatibility with living bone to be acceptable materials for use in making surgically implantable prosthetic devices when the site of installation is properly prepared to receive them. There is, however, less certainty about the ideal physical properties of the surfaces of the prosthetic devices which confront the host bone.
Accordingly, at the present time, owing to its predictable success, the endosseous dental implant fixture made of titanium is the artificial root most frequently chosen for restoring dentition to edentulous patients. However, that success depends in part on the micromorphologic nature of the surface of the implant fixture which comes in contact with the host bone. The response of cells and tissues at implant interfaces can be affected by surface topography or geometry on a macroscopic basis as well as by surface morphology or roughness on a microscopic basis.
Because there is no standard for surface micromorphology of dental implants, the surfaces of commercial implants have a wide range of available textures. Up to now, it is known that osseointegration of dental implants is dependent, in part, on the attachment and spreading of osteoblast-like cells on the implant surface, and it appears that such cells will attach more readily to rough surfaces than to smooth surfaces, but an optimum surface has yet to be defined. Buser, D. et al "Influence of surface characteristics on bone integration of titanium implants. A histomorphometric study in miniature pigs" Journal of Biomedical Materials Research, Vol. 25, 889-902(1992).
It has been suggested by some that the ideal surface will have pits that have very small dimensions in the nanometer range, and by others that bone growth into a porous surface coat of Ti6AlV4 alloy with pore size between 50 and 150 micrometers is an efficient and rapid means of bonding endosseous dental implants in humans. Buser et al (Ibid) clearly showed that the extent of bone/implant interface is positively correlated with increasing roughness of the implant surface.
Mention of implant topography, or texture, occurs frequently in the literature and patents relating to this subject; for example, U.S. Pat. No. 4,180,910 issued to F. Straumann et al, Jan. 1, 1980, at column 2, lines 59 to 67 describes a dental implant member made of titanium provided with a coating of titanium granules applied to it by flame spraying, producing a rough surface "comprising pores, most of which have a diameter of 1 to 10 um, although still smaller pores are also present". Similar results are reported in a published article emanating from the University of Bern (Switzerland), Clinic for Dental Maintenance (Schweizerische Monatschrift fur Zahnheilkunde, Vol. 86, No. 7, July 1976, pp. 713-727). U.S. Pat. No. 3,855,638 issued to Robert M. Pilliar Dec. 24, 1974 discloses a surgical prosthetic device made of a body of a selected material and having on its surface a layer of particles of the same material forming a porous coat of the particles. It has been suggested, e.g: PCT application No. SE91/00672, International Publication No. WO92/05745 and date Apr. 16, 1992, to blast the surface of implants of titanium or an alloy of titanium with particles of a titanium oxide.
Prior-known processes for achieving biocompatible surfaces on surgically implantable prosthetic devices have taken many forms, classifiable primarily as:
1--processes which add something to the surface; and PA1 2--processes which take something away from the surface.
Among the first class are coating, cladding and plating processes, which include, as examples, the processes described in the above-mentioned patents to Straumann et al, and to Pilliar, and processes which coat a body with bone-compatible apatite materials, such as hydroxyapatite or whitlockite. The latter has spawned a vast literature which includes U.S. Pat. No. 4,818,559 issued to Hama et al. Apr. 4, 1989. Among the second class are acid etching, ion etching, chemical milling, laser etching and spark erosion. Sometimes processes of both classes are combined, as in the Hama et. al. patent.
It is well understood that the chemical properties of the surfaces of surgically implantable prosthetic devices should as much as possible exclude impurities which could lessen the biocompatibility of the devices. See, Ibid, Pilliar and PCT publication No. WO92/05745. Thus, for example, traces of elements of the tools used to turn titanium in a lathe may become embedded in the surface of a device, contaminating the device notwithstanding that the starting piece of titanium may have been "pure" to the governing specification of biocompatibility.
It has been suggested that the type of roughness produced on a chemically pure titanium surface does affect initial biological responses such as cellular attachment and spreading. An irregular rough surface produced by sand blasting appears to be more conducive to cellular attachment than other rough surfaces produced by polishing/grinding or acid etching. (Int. J Oral Maxillofacial Implants 1992;7: 302-310)
P. Ducheyne et al, "The Effect of Hydroxyapatite Impregnation on Skeletal Bonding of Porous Coated Implants" J. Biomed. Mater. Res. 14,225-237 (1980) describe a cylindrical plug of stainless steel coated with a porous metal fiber of the same material, with the individual fibers lined with hydroxyapatite, for increasing the rate of bone in growth when implanted in the femur of living dogs.
S. D. Cook et al, "Torsional Stability of HA Coated and Grit-Blasted Titanium Dental Implants" J. Oral Implantal. 18(4)pp 354-65 (1992), in a comparative study in dogs, showed superior interface torsional strength for the HA-coated implants, with interface failure seen to occur primarily at the HA/implant interface.
The present invention is not based essentially on addition to or taking away from the surface of the prosthetic device. While it may fit incidentally into the second class, and in some embodiments it may appropriate properties of the first class, it is primarily concerned with reforming the surface without necessarily removing any material from it or adding any material to it. The invention accordingly addresses new and greatly improved processes for achieving a biocompatible surface on surgically-implantable devices wherein the existing surface is selectively and controllably manipulated to form a new prescribed surface topography or texture.